> They control the Steam distribution network.

Tautologically true and therefore irrelevant. That's exactly the same as saying "Walmart has a monopoly over Walmart's physical stores" - that's not a meaningful statement and it has nothing to do with either monopoly status or consumer harm.

> It may not be physical

...and therefore it's categorically different. Don't be dishonest.

> you still have to use it to reach a large portion of PC Gamers

It's called a "distribution channel". You only "have" to use it, in the sense that most people look for stuff in Steam before they do anywhere else, but it is factually different than a telecom monopoly, where you cannot get internet from more than one provider in your neighborhood. This comparison is irrelevant and highly dishonest.

> due to network effects

No, network effects are secondary. People do not install Steam because their friends are there, they install Steam because they want to buy a game or download the games they already have. That's not "network effects" - that's using the tool.

> no one wanting to run multiple clients

Also untrue - almost every single person that I know uses multiple clients, and I've only ever once heard someone refuse to install an additional client, and it was on principle (Epic Games).

> Currently you have to also make use of their other services like the Store, and pay for them with a large sales cut, in order to use the distribution network, no matter if you want those services or not.

...and because it's one of the largest digital distribution networks in the world, this is entirely fair.

You're very clearly trying to stretch the definition of "monopoly" and manufacture harm, without actually knowing anything about Steam or how people use it.